Olefins are produced in a feed gas that originates from catalytic reactors and/or thermal cracking furnaces that are well known in the art, such as, for example, the SUPERFLEX™ process of Kellogg Brown & Root LLC, the catalytic process for cracking methanol, the deep catalytic cracking process, FCC reactors, and the like. The olefin containing feed gas must be processed to separate and recover the olefins from various amounts of other gases, which can include hydrogen, methane, ethane, propane, butanes, and pentanes-and-heavier hydrocarbons. The feed gas can also include contaminants such as carbon dioxide, acetylene, arsenic, mercury, carbonyl sulfide, nitrogen oxides, oxygen, and the like, which must generally be removed or treated.
In some conventional olefin recovery processes, the feed gas is compressed and fractionated in a front-end, heat-pumped deethanizer or depropanizer, employing relatively high pressures on the order of 400-500 psia in the case of the front end deethanizer and 400-600 psia in the case of the front end depropanizer. Schematic diagrams showing these prior art olefin recovery schemes are illustrated in FIGS. 1 and 2. Front-end demethanizer processes have also been used, employing pressures of 500-600 psia. High pressures are required in these olefin recovery schemes to obtain high ethylene/propylene recoveries. These relatively high pressures typically require four compressor stages, and an expander-recompressor is employed around the cold section refrigeration system. The high pressure of the equipment and the number of pieces of equipment increases the capital cost of the equipment. It would be desirable to reduce the number of pieces of equipment, as well as the cost.
In the processing of feed gases containing trace amounts of nitrogen oxides, such as, for example, in FCC effluent, there is also a potential safety hazard that must also be considered. A reactor is normally used to remove nitrogen oxides before the process gas is sent to the cold section, but leakage or upset or other malfunction can result in nitrogen oxides being present in the cold section. The presence of nitrogen oxide at temperatures below about 105° C. can result in the formation and accumulation of nitrated gums in the coldest cold box exchanger. Nitrated gums are unstable and can explode if thermally or mechanically shocked. Temperatures below 105° C. in the cold box should be avoided to minimize the possibility of nitrated gum formation.
One goal is to maximize ethylene/propylene recovery while at the same time minimizing energy consumption and other operating costs. Often, the lower temperatures required to reduce the loss of olefin in tail gas and/or hydrogen product streams will require additional power, creating a trade-off between power consumption and olefin losses. To maximize heat and refrigeration recovery, a relatively large number of heat exchangers may be employed. Furthermore, higher olefin recovery rates can necessitate the use of colder temperatures below the temperature at which nitrated gums can form in the cold box.
The present embodiments are detailed below with reference to the listed Figures.